query(p44,w(_684200),[w(a),w(b),w(c),w(d)],[w(c)],[w(a),w(b)]).
%-----------------------------------------------------------

:- table w / 1.

w(A) :- m(A,B), tnot(w(B)).
m(a,b).
m(b,a).
m(b,c).
m(c,d).
